Inkjet recording apparatus

Abstract: An inkjet recording apparatus comprises an inkjet head having formed in a p-type or n-type semiconductor substrate (1) at least one nozzle (4), an ink supply passage (6, 7) connected to the nozzle and a diaphragm (5) contiguous to the ink supply passage, and further having electrostatic actuator means for driving the diaphragm such that in response to a diaphragm motion ink droplets are ejected from the nozzle, said electrostatic actuator means comprising a first electrode (17) attached to the substrate, said diaphragm (5) and a second electrode (21) disposed opposite to said diaphragm with a gap (G) therebetween; and drive circuit means (102) for selectively applying a drive voltage between said first and second electrodes (17, 21), wherein the polarity of said drive voltage is selected such that in case of a p-type substrate said first electrode is made positive with respect to the second electrodes, while in case of an n-type substrate said first electrode is made negative with respect to the second electrodes. For ejecting an ink droplet from the nozzle the electrostatic actuator is first charged and then discharged.

Description: The present invention relates to an inkjet recording apparatus having a so-called ink-on-demand type inkjet head which ejects ink droplets only when respective dots are actually to be recorded on a recording medium. More specifically, the invention relates to an inkjet recording apparatus having an electrostatically driven inkjet head. Mainly two kinds of ink-on-demand type inkjet heads are currently used differing in the way of generating the pressure required for ink ejection. One kind uses piezoelectric actuators for this purpose as disclosed in, e.g., DE-A-31 47 107 and EP-A-0 337 429, while the other employs heating elements for heating ink so as to generate bubbles as described in, e.g., JP-B-59911/1986. Each of these two kinds of inkjet head has its own merits and demerits. While the former type suffers from problems in manufacturing when a certain nozzle density and precision is required, it enjoys a high reliability and a long service life. On the other hand, the bubble type inkjet head presents less manufacturing problems but its resistive heating elements tend to become damaged over time as a result of the repeated rapid heating and cooling and the impacts caused by collapsing bubbles, and so the practical service life of the inkjet head is accordingly short. Thus, none of these two kinds of inkjet heads is really fully satisfactory. A third known principle for pressure generation in an inkjet head makes use of an electrostatic force, i.e., employs an electrostatic actuator as disclosed in JP-A-289351/1990 and US-A-4,520,375. More particularly, JP-A-289351/1990 discloses an inkjet head comprising a silicon substrate having formed therein ink passages each connected to a respective nozzle at one end and to a common ink reservoir at the other end. A side wall portion of the ink passage is formed by a diaphragm as a vibration plate. A respective individual or nozzle electrode is provided on the outside surface of each diaphragm. Disposed opposite the nozzle electrodes, via a gap, is a common electrode. Each diaphragm with its nozzle electrode and the opposing common electrode constitutes an electrostatic actuator including a capacitor formed by the nozzle electrode, the common electrode and an insulator therebetween. A similar electrostatic actuator or fluid jet ejector is disclosed in US-A-4,520,375. In this latter prior art, by utilizing its semiconducting property, the thin silicon diaphragm itself forms one electrode of the capacitor. Impressing a time varying voltage on the capacitor causes the diaphragm to be set into mechanical motion and the fluid to exit responsive to the diaphragm motion. The electrostatic principle utilized in this prior art offers advantages such as compactness, high density, and a long service life, and, therefore, appears to be a promising alternative by which the above stated problems of the prior art using either piezoelectric actuators or heating elements may be resolved. The document EP-A-0 479 441 discloses an inkjet recording apparatus according to the precharacterizing clause of claim 1. In this prior art, drive means include an oscillation circuit for alternately generating a zero voltage and a positive voltage or an AC electric source to selectively drive each actuator in response to a print control signal. In an alternative embodiment EP-A-0 479 441 discloses a structure of an electrostatic actuator in which the diaphragm itself does not form one of two capacitor electrodes to which the drive voltage is applied. Instead, two separate electrodes are disposed side by side. One or both electrodes are located opposite the diaphragm and positive and negative pulses are alternately applied to these two electrodes, one pulse for attracting the diaphragm to prepare for ejection of an ink droplet and the following pulse of opposite polarity for repulsing the diaphragm and to effect the ejection of the ink droplet. However, a practical implementation of an ink-on-demand type inkjet head using such electrostatic actuators for pressure generation and featuring high quality printing and constant high efficiency has not been possible yet. The use of semiconductor material for at least part of such inkjet head including the diaphragm allows complex structures to be manufactured with high precision by employing well known semiconductor processing technologies. Thus, the demand for a high nozzle density at relatively low manufacturing costs can be met. On the other hand, problems are incurred by the semi-conducting nature of the material in that it is difficult to assure stable drive characteristics. Therefore, the object of the present invention is to provide an inkjet recording apparatus having an inkjet head driven by means of an electrostatic force wherein the inkjet head can be easily and precisely manufactured and a stable ink supply and good print quality is assured. This object is achieved with a recording apparatus as claimed in claim 1. Preferred embodiments of the invention are subject-matter of the dependent claims. The inkjet head of a recording apparatus in accordance with the present invention comprises a substrate made from a p-type or n-type doped semiconductor material and having formed therein one or plural nozzles for ejecting ink droplets and an ink chamber connected to each nozzle and forming part of an ink supply passage for supplying ink to each nozzle. At least one wall of each ink chamber is formed by a diaphragm integral with the semiconductor substrate. An individual nozzle electrode is positioned opposite each diaphragm with a gap therebetween, and a common electrode is formed on the substrate. The diaphragm and the nozzle electrode corresponding to it constitute a capacitor which, due to the flexible nature of the diaphragm, acts as an electrostatic actuator. If the capacitor is charged an electrostatic force will deflect the diaphragm towards the nozzle electrode. Upon discharging the capacitor the diaphragm will return to its initial state due to its resilience. With such structure of the inkjet head, when a voltage is applied to the common electrode and a nozzle electrode a difference in the resulting deflection of the diaphragm depending on the polarity of the voltage may be observed. This is attributed to the MIS structure formed between the nozzle electrode, the insulating gap between the nozzle electrode and the diaphragm, and the diaphragm itself. In this structure a space-charge layer (also known as "depletion layer") is created in the semiconductor material at the interface to the insulator. The space-charge layer has a certain capacitance which is connected in series to the capacitor formed by the diaphragm and the nozzle electrode. Depending on the capacitance of the space-charge layer a smaller or larger proportion of the voltage applied between the common electrode and the nozzle electrode will be effective for establishing the electrostatic field that causes displacement of the diaphragm. According to the present invention the polarity of the voltage applied between the common electrode and the nozzle electrodes is selected depending on the kind of semiconductor material such as to substantially suppress the influence of a space-charge layer. With a p-type semiconductor substrate this is achieved by having the polarity of the voltage so that the potential of the common electrode is positive with respect to that of the nozzle electrodes. With an n-type substrate the polarity of the voltage is reversed. In an inkjet recording apparatus according to the present invention, a pulse voltage is applied between the common electrode and the nozzle electrodes for charging the electrostatic actuators, i.e., for generating an electrostatic force causing an attraction between the diaphragm and the nozzle electrode positioned opposite the diaphragm and deflecting the diaphragm. By then discharging the electrostatic actuator, i.e., by cancelling the electrostatic force, the pressure inside the ink chamber is increased due to the restoring force of the diaphragm, and ink is ejected from the nozzle. By selecting the polarity of the pulse voltage applied to the common electrode and the nozzle electrodes according to the conductivity type of the semiconductor used for the substrate, and by charging and discharging the electrostatic actuator as described above, it is possible to control the deflection of the diaphragm in response to the applied voltage in an extremely narrow range, thus suppressing diaphragm displacement defects and variations, stabilizing the ink ejection speed and volume, and obtaining extremely high quality printing. The effective service life of the diaphragm is also increased by a stable diaphragm drive, and the ink ejection reliability is improved. Preferred embodiments of the present invention are described below with reference to the accompanying drawings, in which: Fig. 1 is a partially exploded perspective view of the inkjet head of a preferred embodiment of the invention, Fig. 2 is a side cross-section of the inkjet head shown in Fig. 1, Fig. 3 is a sectional view from line A-A in Fig. 2, Fig. 4 is a schematic diagram illustrating the charge distribution in the diaphragm and the nozzle electrode arrangement when a voltage whose polarity is selected in accordance with the invention is applied, Fig. 5 is a schematic diagram illustrating the charge distribution in the diaphragm and nozzle electrode when the polarity of the drive voltage is opposite to that used in Fig. 4, Fig. 6 is a schematic diagram of a drive circuit according to the present invention, Fig. 7 is a drive timing chart, Fig. 8 shows the inkjet head state during electrostatic charging, Fig. 9 shows the inkjet head state during electrostatic discharging, and Fig. 10 is a conceptual diagram of an inkjet recording apparatus embodying the invention. Fig. 1 is a partially exploded perspective view and cross-section of a preferred embodiment of the inkjet head of a recording apparatus embodying the present invention. Note that while this embodiment is shown as an edge type head wherein ink is ejected from nozzles provided at the edge of a substrate, the invention may also be applied to a face type head wherein the ink is ejected from nozzles provided on the top surface of the substrate. Fig. 2 is a side cross-section of the assembled inkjet head, and Fig. 3 is a sectional view from line A-A in Fig. 2. The inkjet head 10 of this embodiment is made up of three substrates 1, 2, 3 one stacked upon the other and structured as described in detail below. A first substrate 1 is sandwiched between second and third substrates 2 and 3, and is made from a silicon wafer. Plural nozzles 4 are formed between the first and the third substrate by means of corresponding nozzle grooves 11 provided in the top surface of the first substrate 1 such as to extend substantially in parallel at equal intervals from one edge of the substrate. The end of each nozzle groove opposite said one edge opens into a respective recess 12. Each recess in turn is connected via respective narrow grooves 13 to a recess 14. In the assembled state the recess 14 constitutes a common ink cavity 8 communicating via orifices 7 formed by the narrow grooves 13, and ink chambers 6 formed by the recesses 12 with the nozzles 4. In the present embodiment, each orifice 7 is formed by three parallel grooves 13 mainly to increase the flow resistance but also to keep the inkjet head operative if one of the grooves becomes clogged. Electrostatic actuators are formed between the first and the second substrate. The bottom of each ink chamber 6 comprises a diaphragm 5 formed integrally with the substrate 1. As will be understood, the grooves and recesses referred to above can be easily and precisely formed by photolithographic etching of the semiconductor substrate. A common electrode 17 is provided on the first substrate 1. The magnitude of the work function of the semiconductor forming the first substrate 1 and the metal used for the common electrode 17 is an important factor determining the effect of electrode 17 on first substrate 1. The semiconductor material used in this embodiment has a resistivity of 8 ∼ 12 Ωcm, and the common electrode 17 has in fact a two-layer structure made from platinum on a titanium base layer or gold on a chrome base layer. In Figs. 4 and 5 17a denotes the upper layer (platinum or gold) and 17b denotes the lower or base layer (titanium or chrome) the latter being provided mainly to improve the bonding strength between the substrate and the electrode. The present invention shall not be so limited, however, and various other material combinations may be used according to the characteristics of the semiconductor and electrode materials. Borosilicate glass, such as Pyrex glass, is used for the second substrate 2 bonded to the bottom surface of first substrate 1. Nozzle electrodes 21 are formed on the surface of second substrate 2 by sputtering gold to a 0.1 µm thickness in a pattern essentially matching the shape of diaphragms 5. Each of nozzle electrodes 21 comprises a lead member 22 and a terminal member 23. A 0.2 µm thick insulation layer 24 for preventing dielectric breakdown and shorting during inkjet head drive is formed from a Pyrex sputter film on the entire surface of the second substrate 2 except for the terminal members 23. In addition or as an alternative to the insulation layer 24 an insulation layer (26 in Fig. 4) may be provided on the side of the diaphragms 5 facing the nozzle electrodes. Since the diaphragms 5 consist of a semiconductor material such insulation layer may be easily formed to a thickness of 0.1µm to 0.2µm by oxidizing the semiconductor material. Such oxide insulation layer exhibits excellent mechanical strength, insulation performance and chemical stability and substantially reduces the possibility of a dielectric breakdown in case of a contact between the diaphragm and the nozzle electrode. This is an advantage of using the semiconductor material itself as an electrode of the electrostatic actuator. A recess 15 for accommodating a respective nozzle electrode 21 is provided below each diaphragm 5. Bonding the second substrate 2 to the first substrate 1 results in vibration chambers 9 being formed at the positions of recesses 15 between each diaphragm 5 an the corresponding nozzle electrode 21 opposite to it. In this embodiment, recesses 15 formed in the bottom surface of the first substrate 1 provide for gaps between the diaphragms and the respective electrodes 21. The length G (see Fig. 2; hereinafter the "gap length") of each gap is equal to the difference between the depth of recess 15 and the thickness of the electrode 21. It is to be noted that this recess can be alternatively formed in the top surface of the second substrate 2. In this preferred embodiment, the depth of recess 15 is 0.6 µm, and the pitch and width of nozzle channels 11 are 0.72 mm and 70 µm, respectively. As with second substrate 2, borosilicate glass is used for the third substrate 3 bonded to the top surface of first substrate 1. Bonding third substrate 3 to first substrate 1 completes formation of nozzles 4, ink chambers 6, orifices 7, and ink cavity 8. An ink supply port 31 is formed in third substrate 3 so as to lead into ink cavity 8. Ink supply port 31 is connected to an ink tank (not shown in the figure) using a connector pipe 32 and a tube 33. First substrate 1 and second substrate 2 are anodically bonded at 300°C to 500°C by applying a voltage of 500 V to 800 V, and first substrate 1 and third substrate 3 are bonded under the same conditions to assemble the inkjet head as shown in Fig. 2. After bonding the substrates, gap length G between diaphragms 5 and nozzle electrodes 21 is 0.5 µm in this embodiment. The distance G1 between diaphragms 5 (or the insulation layer 26, if any) and insulation layer 24 covering nozzle electrodes 21 is 0.3 µm. The thus assembled inkjet head is driven by means of a drive unit 102 connected by leads 101 to common electrode 17 and terminal members 23 of nozzle electrodes 21. Drive unit 102 includes a plurality of drive circuits 40 (see Fig. 2), one for each actuator. Ink 103 is supplied from the ink tank (not shown in the figures) through ink supply port 31 into first substrate 1 to fill ink cavity 8 and ink chambers 6. Also shown in Fig. 2 is an ink droplet 104 ejected from nozzle 4 during inkjet head drive, and recording paper 105. The electrical connections of the present embodiment are described next. Due to the MIS structure referred to above there may be a large difference in the current value depending on the polarity of the applied voltage because of the effect of the space-charge layer. When the semiconductor used for the substrate is p-type silicon, the substrate acts as a conductor when, relative to the nozzle electrodes 21, a positive potential is applied to the common electrode 17, but when a negative potential is applied, the substrate does not act as a conductor and instead a space-charge layer is created. This characteristic is used in the present invention, which is described below with reference to Figs. 4 ∼ 10. Fig. 4 is a schematic view illustrating the distribution of electric charges in the diaphragm and the nozzle electrode when the polarity of the applied voltage is selected in accordance with the present invention. A p-type silicon is used for first substrate 1 in this embodiment and the common electrode 17 and the nozzle electrodes 21 of the electrostatic actuators are connected to drive circuits 40 so that for charging an actuator a pulse voltage is applied by which the common electrode is rendered positive with respect to the nozzle electrode 21. The p-type silicon is doped with acceptor impurities such as boron and has as many holes as the number of acceptor atoms. The pulse voltage establishes an electrostatic field directed from the diaphragm to the nozzle electrode. Because of this field the holes 19 in the p-type silicon migrate towards insulation layer 26 leaving negatively charged acceptor ions. Because holes are injected from the common electrode 17 the negative charge of the acceptor ions is neutralized. Therefore, the diaphragm assumes a positive charge with no space-charge layer being created, i.e. the diaphragm or the first substrate functions as a conductor. In addition, a negative charge accumulates on the nozzle electrodes 21 side. As a result, the pulse voltage applied between a diaphragm 5 and its opposing nozzle electrode 21 generates an attractive force, due to static electricity, sufficient to deflect diaphragm 5 towards the nozzle electrode 21. Fig. 5 is a view similar to Fig. 4 and illustrates the distribution of electric charges in the diaphragm and the nozzle electrode when the polarity of the applied voltage is opposite to that in Fig. 4, i.e., the first substrate 1 is negative with respect to the nozzle electrodes 21 . In this case, since the electrostatic field is directed from the nozzle electrode to the diaphragm, holes 19 in the p-type silicon diaphragm 5 migrate towards the common electrode 17. Because the acceptor atoms are fixed to the silicon crystals and cannot move and no holes can be injected from the nozzle electrode into the diaphragm, the silicon is electrically separated into two layers, a first layer positively charged by holes 19 and a second layer negatively charged by acceptor ions. As a result, first substrate 1 has a capacitance determined by the depth of space-charge layer 25 and the dielectric constant of the silicon, and therefore functions as a capacitor. First substrate 1 therefore does not function as a conductor, and the electrostatic attraction force produced between a diaphragm 5 and an opposing nozzle electrode 21 decreases relative to the applied pulse voltage by an amount equivalent to the voltage drop across the capacitance. As a result, diaphragm 5 does not deflect sufficiently, and inkjet performance cannot be assured. There are also cases in which deflection of diaphragm 5 becomes impossible, and it is not possible to drive the inkjet head when the first substrate 1 is used as the negative electrode. When an n-type silicon semiconductor is used for the substrate material the situation is opposite to the one explained above, i.e., when a negative potential is applied to the substrate 1, the substrate operates as a conductor while when a positive potential is applied to the substrate 1 the substrate does not become a conductor and has capacitance due to the space-charge layer, wherein mobile electrons do not exist. As a result, this n-type silicon semiconductor substrate can be driven identically to a p-type semiconductor by applying a voltage with the polarity opposite to that applied with a p-type semiconductor substrate, and good inkjet performance can be assured. Fig. 6 is a schematic diagram of a drive circuit 40 according to the present invention. A capacitor 110 shown in the figure, represents the capacitor formed by a diaphragm 5 and the corresponding one of the electrodes 21. 106 and 107 are a first and a second switching element which may both be a bipolar transistor or MOS transistor. Note that the drive circuit as described herein assumes an n-type substrate material. As will be clear from the foregoing, when an n-type semiconductor is used for the substrate 1, the configuration of the drive circuits 40 and/or the connections between drive circuits 40 and the inkjet head 10 will be such that the polarity of the voltage applied to each actuator is opposite to that applied in case of a p-type semiconductor. Fig. 7 is a timing chart of electrostatic charging and discharging, Fig. 7 (A) showing the charge timing, and Fig. 7 (B) the discharge timing. The charge signal 111, discharge signal 112, and charge time T are also shown. Fig. 8 and Fig. 9 show the inkjet head state during electrostatic charging and discharging, respectively, in the present embodiment. The drive method of the present embodiment structured and connected as described above is described below. When the ink eject signal, i.e. charge signal 111, is input to a gate 121 of drive circuit 40, transistor 108 becomes ON, first switching element 106 becomes ON, and the capacitor 110 is charged via a resistor 113. Electrostatic attraction thus causes diaphragm 5 to deflect towards nozzle electrode 21 as shown in Fig. 8. The deflection of the diaphragm enlarges the volume of the ink chamber 6 and causes ink to be taken in from the ink cavity through the orifice 7. After a predetermined charge time T has passed, the trailing edge of the charge signal causes transistor 108 to return to an OFF state thereby rendering the first switching element 106 OFF and interrupting the charging path for capacitor 110. Then a discharge signal 112 is input to a gate 122 causing transistor 109 to become OFF and second switching element 107 ON. In response to that, current flows in the direction of arrow A (Fig. 6) through a resistor 114, and the charge stored in the capacitor 110 is suddenly discharged. As a result, the attractive force caused by static electricity acting on nozzle electrode 21 and diaphragm 5 dissipates, and diaphragm 5 returns to the original position due to its inherent rigidity. This causes the pressure inside ink chamber 6 to rise rapidly, and ink droplet 104 is ejected from nozzle 4 to recording paper 105 (Fig. 9). By appropriately selecting the resistance values of resistors 113 and 114 desired charge/discharge characteristics may be obtained with a relatively slow charge speed and a fast discharge speed. Fig. 10 shows an overview of a printer as an example of an inkjet recording apparatus that incorporates the inkjet head described above. 300 denotes a platen as a paper transport means that feeds recording paper 105. 301 indicates an ink tank that stores ink in it and supplies ink to the ink jet head 10 through an ink supply tube 306. The ink jet head 10 is mounted on a carriage 302 which is movable by means of carriage drive means (not shown) in a direction perpendicular to the direction in which the recording paper 105 is transported. In synchronisation with the movement of the ink jet head, ink droplets are selectively ejected from a row of nozzles so as to print characters and/or graphics on the recording paper 105. In the printer of Fig. 10 a device is provided for preventing the clogging of the ink jet head nozzles, a problem peculiar to printers that incorporate on-demand-type ink jet heads. To prevent the clogging of the nozzles, the ink jet head is moved to a position in front of a cap 304, and then ink discharge operations are performed several times while a pump 303 is used to suction the ink through the cap 304 and a waste ink recovery tube 308 into a waste ink reservoir 305. In printing tests using the drive method and the printer described above, the printer was successfully driven with a 50 V low voltage power supply, and ink was stably delivered to the paper with good print quality up to 5 kHz at an inkjet volume of 0.15 µm 3 and an inkjet output rate of 10 m/sec. This inkjet head drive method was also confirmed to offer excellent durability with a minimum of two billion inkjet eject repetitions.

Claims: An inkjet recording apparatus comprising an inkjet head having formed in a p-type or n-type semiconductor substrate (1) at least one nozzle (4), an ink supply passage (6, 7) connected to the nozzle and a diaphragm (5) contiguous to the ink supply passage, and further having electrostatic actuator means comprising a first electrode (17) attached to the substrate, said diaphragm (5) and a second electrode (21) disposed opposite to said diaphragm with a gap (G) therebetween, and drive circuit means (102) for selectively applying a drive voltage between said first and second electrodes (17, 21) so as to move said diaphragm, such that in response to a diaphragm motion ink droplets are ejected from the nozzle, characterized in that , the polarity of said drive voltage is selected such that in case of a p-type substrate said first electrode is made positive with respect to the second electrode, while in case of an n-type substrate said first electrode is made negative with respect to the second electrode. The apparatus of claim 1, wherein said drive circuit means comprises means for generating a charge pulse signal of a predetermined charge pulse width and a discharge pulse signal of a predetermined discharge pulse width, means for applying a first potential to the first electrode (17), first switch means (106, 108, 121) responsive to said charge pulse signal for applying a second potential to the second electrode during said charge pulse width, and second switch means (107, 109, 122) responsive to said discharge pulse signal for applying said first potential to the second electrode during said discharge pulse width. The apparatus of claim 2, wherein said drive circuit means further comprises means (113) for determining the charging speed and means (114) for determining the discharging speed of the electrostatic actuator.

Inventors: FUJII MASAHIRO, KOBAYASHI NAOKI, KOEDA HIROSHI, MIYASHITA IKUHIRO, SUGIMURA SHIGEO, FUJII, MASAHIRO, KOBAYASHI, NAOKI, KOEDA, HIROSHI, MIYASHITA, IKUHIRO, SUGIMURA, SHIGEO, FUJII, MASAHIRO, C/O SEIKO EPSON CORPORATION, KOBAYASHI, NAOKI, C/O SEIKO EPSON CORPORATION, KOEDA, HIROSHI, C/O SEIKO EPSON CORPORATION, MIYASHITA, IKUHIRO, C/O SEIKO EPSON CORPORATION, SUGIMURA, SHIGEO, C/O SEIKO EPSON CORPORATION
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1994
Main classification: B41J2
IPC codes: B41J2